Guide numbers
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Simple.
Divide the guide number by the distance and you get the aperture you need.
So if a flash has a guide number of 120 feet at ISO 100 (they are usually quoted at ISO 100) and you want to use it eight feet away from the subject, the aperture you need is 120/8 = 15. Nearest real aperture setting of f16 is close enough.
To test the theory that if you double the distance, the light level is reduced to 1/4, lets try it at 16 feet.
120/16 = 7.5. close enough to f8 which is two stops away from f16 and therefore, 1/4 of the light.
Steve.
Divide the guide number by the distance and you get the aperture you need.
So if a flash has a guide number of 120 feet at ISO 100 (they are usually quoted at ISO 100) and you want to use it eight feet away from the subject, the aperture you need is 120/8 = 15. Nearest real aperture setting of f16 is close enough.
To test the theory that if you double the distance, the light level is reduced to 1/4, lets try it at 16 feet.
120/16 = 7.5. close enough to f8 which is two stops away from f16 and therefore, 1/4 of the light.
Steve.
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As usual, Wiki puts it into better words that what I could!
http://en.wikipedia.org/wiki/Guide_number
And IIRC, this lot should even be accurate!
http://strobist.blogspot.co.uk/2007/12/guide-number-your-free-flash-meter.html
http://en.wikipedia.org/wiki/Guide_number
And IIRC, this lot should even be accurate!
http://strobist.blogspot.co.uk/2007/12/guide-number-your-free-flash-meter.html
OP
SpikeK6
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Thanks steve that was an excellent explanation.
Thanks Nod will take a look at these too
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In Ye Olde days, people didn't have flash meters. They were very rare and 'normal' people couldn't afford them.
Guide numbers are an easy way to work out the aperture you need and are no different to the calculator dial or table on the back of most older flashes.
The guide number was originally used for flash bulbs. My first camera was an Agfa Isolette. I had a flash for this which took bulbs. The packet of bulbs gave you the guide number and usually some examples or a table to help you work it out.
Like this: http://farm5.staticflickr.com/4124/5217698929_dbea289292_z.jpg
Steve.
Guide numbers are an easy way to work out the aperture you need and are no different to the calculator dial or table on the back of most older flashes.
The guide number was originally used for flash bulbs. My first camera was an Agfa Isolette. I had a flash for this which took bulbs. The packet of bulbs gave you the guide number and usually some examples or a table to help you work it out.
Like this: http://farm5.staticflickr.com/4124/5217698929_dbea289292_z.jpg
Steve.
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Yes, guide numbers are used to state flash power output, but they are not very reliable and are hard to compare between brands accurately, though it's often the best indicator you'll get.
In particular, the angle of light coverage quoted, or with studio heads the type of reflector fitted. For example, Canon 580EX has a stated guide number of 58 (ISO100/metres) and hence the model designation. 430EX is GN43.
But these figures refer to the zoom head set at longest focal length, eg 105mm. At 50mm, the 580EX drops to GN42, at 24mm it's GN28, and with the 14mm wide panel flipped down, it's only GN15. That's a difference of four stops from one end to the other - massive change.